"Nothing has changed about the missions that we're conducting inside Iraq. Airstrikes are authorised under two mission areas -- humanitarian assistance and the protection of US personnel and facilities," Xinhua quoted John Kirby, press secretary of the US Defence Department as saying during a press briefing.
The air strikes in and around Mosul Dam fit into both of those categories, he said.
"We believed that, should the dam remain in control of ISIL(now known as IS) -- whose intentions are obviously not perfectly clear and certainly not in the best interests of the people of Iraq -- if that dam was to blow or they were to open and flood the gates, that it could have an effect as far south as Baghdad," the official said.
He added that Iraqi and Kurdish forces continue to hold the dam, the country's largest which provides water, electricity and flood control for Mosul's 1.7 million residents.
The dam's location and precarious condition meant that its possession by IS forces posed a threat to US personnel and facilities in Baghdad. If the dam were to fail or be sabotaged, the resulting damage would rise to the level of a humanitarian disaster, he said.
A 2009 paper by Mosul University geologists estimated that up to 54 percent of Mosul would be under a maximum of 83 feet (25.3 metres) of water if the dam was breached.